In many applications a power converter is required to provide a voltage within a predetermined range formed from a voltage source having a different voltage level. Some circuits are subject to uncertain and undesirable functioning and even irreparable damage if supplied power falls outside a certain range. More specifically, in some applications, a precise amount of power is required at known times. This is referred to as regulated power supply.
In order to control a power converter to deliver a precise amount of power as conditions require, some form of control of the power converter is required. This control can occur on the primary side of an isolation transformer or the secondary side. A closed loop feedback control system is a system that monitors some element in the circuit, such as the circuit output voltage, and its tendency to change, and regulates that element at a substantially constant value. Control on the secondary side of a power converter can use a monitored output voltage as feedback control, but requires the use of some communication from the secondary to the primary side of the isolation transformer to control the primary side switching element. Control on the primary side can readily control the primary side switching element, but requires some feedback mechanism from the secondary side to the primary side to convey the status of the monitored element. In some applications, an optical coupler circuit, or opto coupler, is used to transmit feedback signals while maintaining electrical isolation between the primary and secondary sides.
FIG. 1 illustrates a conventional regulated switch mode power converter including an optical coupler circuit. The power converter 2 is configured as a traditional flyback type converter. The power converter 2 includes an isolation transformer 4 having a primary winding P1 and a secondary winding S1. The primary winding P1 is electrically coupled to an input voltage Vin and a driving circuit including a transistor 8, a resistor 12, and a controller 10. A capacitor 28 is coupled across the input Vin and coupled with the primary winding P1. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. The transistor 8 is a fast-switching device, such as a MOSFET, the switching of which is controlled by the fast dynamic controller 10 to maintain a desired output voltage Vout. The controller 10 is coupled to the gate of the transistor 8. As is well known, the DC/DC conversion from the primary winding P1 to the secondary winding S1 is determined by the duty cycle of the PWM switching signal provided to the transistor 8. The secondary winding voltage is rectified and filtered using the diode 6 and the capacitor 22. A sensing circuit and a load 14 are coupled in parallel to the secondary winding S1 via the diode 6. The sensing circuit includes resistor 16, resistor 18, and a secondary controller 20. A secondary controller 20 senses the output voltage Vout across the load.
In this configuration, the power converter is controlled by driving circuitry on the primary side, and the load coupled to the output is isolated from the control. As such, a monitored output voltage used for voltage regulation is required as feedback from the secondary side to the control on the primary side. The power converter 2 has a voltage regulating circuit that includes the secondary controller 20 and an optical coupler circuit. The optical coupled circuit includes two galvanically isolated components, an optical diode 24 coupled to the secondary controller 20 and an optical transistor 26 coupled to the controller 10. The optical diode 24 provides optical communication with the optical transistor 26 across the isolation barrier formed by the transformer 4. The optical coupler circuit in cooperation with the secondary controller 20 provides feedback to the controller 10. The controller 10 accordingly adjusts the duty cycle of the transistor 8 to compensate for any variances in an output voltage Vout.
However, the use of an optical coupler circuit in and of itself presents issues. Firstly, the optical coupler circuit adds extra cost. In some applications, the optical coupler circuit can add more cost to the power converter than the isolation transformer. The optical coupler circuit also adds to manufacturing and testing costs. Furthermore, the performance of the optical coupler circuit degrades over time and therefore introduces another potential point of failure in the overall power converter. Also, characteristics of the optical coupler circuit must be accounted for in the overall circuit design. For example, the optical diode component is non-linear and as such a correlation between the optical diode and the optical transistor must be established. The optical coupler circuit also has delays related to the operation of the optical diode and the optical transistor, and the operation of the optical diode requires a well defined DC level. As a result, it is generally desirable to avoid the use of an optical coupler circuit.
A next generation of feedback control does not use optical control circuitry. Instead, the transformer is used to convey real-time feedback signaling from the secondary side to the primary side. In such an application, the transformer includes an auxiliary winding on the primary side that is magnetically coupled to the secondary winding. FIG. 2 illustrates a conventional regulated power converter including a magnetically coupled feedback circuit. The power converter 32 is configured as a traditional flyback type converter. The power converter 32 includes an isolation transformer 34 having a primary winding P1 and a secondary winding S1. The primary winding P1 is electrically coupled to an input voltage Vin and a driving circuit including a transistor 44, a resistor 46, and a controller 42. A capacitor 58 is coupled across the input Vin and coupled with the primary winding P1. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. Similar to the power converter in FIG. 1, the transistor 44 is a fast-switching device controlled by the fast dynamic controller 42 to maintain a desired output voltage Vout. The secondary winding voltage is rectified and filtered using the diode 36 and the capacitor 38, with the output voltage Vout delivered to the load 40.
The power converter 32 has a feedback loop that includes a magnetically coupled feedback circuit coupled to the secondary winding S1 of the transformer 34 and the controller 42. The magnetically coupled feedback circuit includes a diode 48, a capacitor 50, resistors 52 and 54 and an auxiliary winding 56. The auxiliary winding 56 is coupled in parallel to the series of resistors 52 and 54.
The voltage VA is proportional to the voltage across the auxiliary winding 56. The voltage VA is provided as a feedback voltage VFB to the controller 42. The current through the transistor 44 is also provided as feedback current IFB to the controller 42. The controller 42 includes a real-time waveform analyzer that analyzes input feedback signals, such as the feedback voltage VFB and the feedback current IFB.
The auxiliary winding 56 is also magnetically coupled to the secondary winding S1. When the current through the diode 36 is zero, the voltage across the secondary winding S1 is equal to the voltage across the auxiliary winding 56. This relationship provides means for communicating the output voltage Vout as feedback to the primary side of the circuit. The voltage across the auxiliary winding 56 is measured when it is determined that the current through the diode 36 is zero, which provides a measure of the voltage across the secondary winding S1 and therefore the output voltage Vout.
The feedback voltage VFB when the diode 36 current is zero is determined and is referred to as the “voltage sense”, and the feedback current IFB when the diode 36 current is zero is determined and is referred to as the “current sense”. The voltage sense and the current sense are input to the real-time waveform analyzer within the controller 42. FIG. 3 illustrates a functional block diagram of a conventional real-time waveform analyzer 60. Error amplifiers 62 and 64 are acceptors of the regulating means, which in this configuration are the sensed voltage and the sensed current. The error amplifier compares the input sensed voltage to a reference voltage and outputs a first difference value. The first difference value is amplified by the gain of the error amplifier 62. The error amplifier 64 compares the amplified first difference value to the sensed current and outputs a second difference value that is either High or Low. A pulse width modulation (PWM) block 66 is configured as a Flip-Flop digital device. The output of the PWM block 66 is set according to the switching frequency of the clock 68 and is Reset by the High or Low value input from the error amplifier 64. The variable signal applied to the Reset pin generates an output signal that is a pulse train modulated by the pulse width. A multiple input OR gate 70 inputs the clock signal, the pulse train signal, a shutdown signal, and a OVP/UVP/OTP signal, where OVP stands for “over voltage protection”, UVP stands for “undervoltage protection” and OTP stands for “over temperature protection”. The waveform analyzer is configured to output a high voltage value when one of the inputs to the OR gate is high or to output a low voltage value when all of the inputs to the OR gate are low. The high voltage value output from the waveform analyzer corresponds to turning on the transistor 44 in FIG. 2. The low voltage value corresponds to turning off the transistor 44. The OR gate also enables a high voltage signal output from the PWM block 66 to propagate to the output by monitoring abnormal conditions such as under voltage, over voltage, over temperature, etc. In this manner, the pulse width of each pulse can be modified which adjusts the output voltage into regulation.
In general, control intricacies of the waveform analyzer are aligned with control argument sampling to achieve overall system functional performance. Sampling argument is in the form of current, voltage and impedance. System functional performance is in the form of pulse width modulation (PWM), pulse frequency modulation (PFM) and pulse amplitude modulation (PAM). The waveform analyzer of FIG. 3 is limited to signal processing in DC or real-time switching waveforms. In either case, the feedback signal received by the waveform analyzer requires some status integrity, such as no noise on the DC level, no disturbance on the switching waveform and to some degree represent a combination of analog and digital representations. The voltage across the auxiliary winding typically forms a pulse train with frequency corresponding to the switching frequency of the driving transistor. The voltage across the auxiliary winding when the secondary winding current is zero, which corresponds to the diode 36 current equaling zero, corresponds to the falling edge of the pulse. As such, measuring an accurate voltage value requires that the pulse is well defined with sufficient pulse integrity particularly at the falling edge. Further, the voltage value immediately following the rising edge includes ringing due to the leakage impedance of the transformer. As such, pulse integrity also requires sufficient time for the voltage value to stabilize following the rising edge. Higher switching frequencies minimize the pulse width and therefore provide less time for voltage stabilization. For at least these reasons, providing a pulse with sufficient pulse integrity is often difficult to achieve.